Continuous casters, such as twin belt casters, single belt casters and recirculating block casters, are commonly used for producing strip ingots (continuous metal strips) from molten metals, particularly aluminum alloys. In casters of this kind, a casting cavity is formed between continuously moving casting surfaces and molten metal is introduced into the casting cavity on a continuous basis. Heat is withdrawn from the metal via the casting surfaces and the metal solidifies in the form of a strip ingot that is continuously withdrawn from the casting cavity by the moving casting surfaces. The heat flux through the casting surfaces (heat extracted from the solidifying metal) must be carefully controlled to achieve cast strip ingots of good surface quality and to avoid distortion of the casting cavity. Different metals (e.g. aluminum alloys) require different levels of heat flux for proper casting on a continuous basis, so it is important to be able to control the casting apparatus to provide the required levels of heat flux for a particular metal being cast.
The primary heat flux control is usually achieved by applying cooling water to the casting belts or blocks. In most belt casters, this is done on the back face of the belt in the region where the belt passes though the casting cavity. However, the heat flux is often adjusted more precisely by additional means. For example, belt casters have been provided with porous ceramic coatings over the metal belts. Such coatings may optionally be partially or completely filled with a high conductivity inert gas, such as helium, to provide further refinement. In such cases, the expense of maintaining a consistent ceramic coating and the cost of the inert gas have made such procedures economically unattractive.
It is also known to apply a layer of a volatile or partially volatile liquid, e.g. an oil, onto the casting surfaces before they come into contact with the molten metal. This layer is often referred to as “belt dressing” or as a “parting layer”. The thickness of the layer can be varied to provide for control of heat flux to the underlying casting surfaces. However, the use of such oils may adversely affect the surface quality of the cast strip ingot (particularly ingots made from aluminum alloys containing high levels of magnesium), and may give rise to environmental issues, particularly when excessive applications are required in order to achieve the desired degree of heat flux control.
U.S. Pat. No. 4,614,224 issued on Sep. 30, 1980 to Paul W. Jeffery et al. and U.S. Pat. No. 6,120,621 issued on Sep. 19, 2000 to Iljoon Jin et al. disclose the use of randomly textured steel belts (textured by means of shot blasting), in which a layer of liquid is applied to the belt surface prior to contacting the surface with the molten metal. The belt surface is cooled by direct application of coolant to the reverse side of the belt as it passes though the casting cavity. The liquid is generally a hydrocarbon which at least partially volatilizes in use and forms a gaseous layer between the molten metal and the belt surface. This gaseous layer has insulating properties and therefore creates a significant temperature drop between the molten metal and the belt surface. The residual liquid itself has relatively little effect. By varying the amount of liquid applied, it is possible to modulate the effect of the gaseous layer and achieve a certain control over the heat flux through the metal belt, and enhance the casting process. These two patents teach a surface roughness of 210 micro-inches (5.3 micrometers) (RMS) and 160 to 512 micro-inches (4 to 13 micrometers) (Ra), respectively.
In casters such as described above where coolant is applied directly to the reverse side of the belt as it passes through the casting cavity, an increased heat flux can be achieved through use of belts of higher conductivity (such as copper), and by reducing the amount of liquid parting layer. Conventional texturing as applied to such high conductivity belts reduces the maximum high heat flux capability, yet elimination of such texturing can lead to problems of meniscus stability during casting.
U.S. Pat. No. 6,063,215 issued on May 16, 2000 to Donald G. Harrington discloses a steel casting belt which is textured in a more regular manner, i.e. it teaches transverse grooves or dimples provided on a steel casting surface. This textured steel belt is then artificially oxidized. The texturing is said to promote a more uniform heat transfer and allow for escape of gases that may form during casting. Such belts are used in casters where the belt is cooled in a area remote from the casting cavity, and does not use a parting agent.
U.S. Pat. No. 6,135,199 issued on 24 Oct. 2000 to Gavin Wyatt discloses a belt caster where the belts may have fine longitudinal grooves, but refers to U.S. application Ser. No. 08/543,445 (which issued by continuation as U.S. Pat. No. 6,063,215) as being the preferred embodiment.
Therefore, there is a need to provide an improved casting belt having a the high heat removal capability characteristic of a casting belt directly cooled by coolant on its reverse face, while providing for a stable casting process with no distortion in the belt.